1. Field of the Invention
The present invention generally relates to improvements in control of an electric power distribution system, and more specifically to the use of intelligent autonomous nodes for isolating faulted sections of distribution lines, reconfiguring, and restoring service to end customers (circuit reconfiguration), and improving circuit protection.
2. Description of Related Art
The power distribution systems of this invention are generally of low to medium-voltage distribution feeders (ranging from approximately 4 KV to 69 KV) originating in power distribution substations and leading to the source of supply for end customers of an electrical supply utility or agency. Although the electrical principles governing operation of these feeders are identical to those governing the operation of the higher voltage generation and transmission systems, the methodologies for building, operating and maintaining the lower voltage systems are different. These methodologies are dictated by much larger quantities and geographical dispersion of distribution equipment, and by much lower quantities of electrical power supplied per mile of circuit. This creates requirements for lower cost, modular, standardized equipment, which can be installed, operated and maintained with minimal labor and human supervision.
Failures of the distribution feeder (faults) occur due to downed power lines, excavation of underground cable or other causes and are typically detectable by sensing excess (short circuit/overcurrent) current, and occasionally by detecting loss of voltage. In distribution systems, it is sometimes the case that a loss of voltage complaint by the customer is the means by which the utility senses the outage, responding by dispatching a crew to isolate the fault and reconfigure the distribution system. The typical devices for isolating these faults are circuit breakers located primarily in distribution substations and fuses located on tap lines or at customer transformers. The substation breakers are generally provided with reclosing relays that cause the breaker to close several times after the breaker has detected an overcurrent condition and tripped open. If during any of these xe2x80x9creclosuresxe2x80x9d, the fault becomes undetectable, service is restored and no extended outage occurs. Particularly on overhead distribution lines, temporary arcing due to wind, lightening, etc causes many faults. Thus, the majority of faults are cleared when the breaker opens and service is restored on the automatic reclose. Alternatively, after some number of reclosure attempts, if the overcurrent condition continues to be present, the recloser goes into a xe2x80x9clockoutxe2x80x9d state which prevents further attempts to clear the fault.
Other than manually operated switches, most distribution feeders have no other means to isolate a fault between the substation and the fuses, thus any failure of the feeder results in lengthy, costly, inconvenient and potentially dangerous outages. The primary exceptions to this involve the use of devices known as xe2x80x9cline reclosersxe2x80x9d, xe2x80x9cinterruptersxe2x80x9d and xe2x80x9cautomatic line sectionalizing switchesxe2x80x9d or xe2x80x9csectionalizersxe2x80x9d. These are automatically operated devices, well known to those skilled in the art, and are referred to categorically in this document as xe2x80x9cfault isolating devicesxe2x80x9d. The reader should be aware that the term xe2x80x9csectionalizerxe2x80x9d refers to a specific family of automatic, fault isolating devices described below, while the terms xe2x80x9csectionalizingxe2x80x9d and sectionalizexe2x80x9d are used to describe the process of isolating a faulted section of line, which can be performed by all of the classes of switches described above.
The xe2x80x9cline recloserxe2x80x9d is typically a pre-packaged, version of the substation breaker with reclosing relay. Line reclosers typically consist of a fault-break switching device with integrated current sensing, plus a control enclosure containing fault detection hardware, control logic, user interface module, and battery-backed power supply. When placed on the distribution line between the substation and customer loads, a line recloser is typically set up with fault detection settings coordinated to operate before the substation breaker trips and to correspondingly prevent the substation breaker from tripping. This has the effect of reducing the number of customers affected by an end of line fault. On very long feeders, the more sensitive settings can be used to protect the feeder from faults of a magnitude too low to be detected reliably by the substation circuit breaker. Multiple line reclosers can be placed on a distribution line in series, although it becomes increasingly difficult or impossible to coordinate their settings such that only the nearest recloser on the source side of the fault operates.
The xe2x80x9cinterrupterxe2x80x9d is typically a pre-packaged breaker and fault relay without automatic reclosing capability. Interrupters are used primarily in underground power distribution systems.
The xe2x80x9cautomatic line sectionalizerxe2x80x9d or xe2x80x9csectionalizerxe2x80x9d is typically a prepackaged combination of a load-break switch used in conjunction with a device known as a xe2x80x9cline sectionalizer controlxe2x80x9d. The sectionalizer senses current (and optionally voltage) such that the operation of the circuit and the source-side protective device can be monitored. The sectionalizer is configured to open its switch under certain circumstances when the circuit is de-energized after some number of pre-configured voltage losses have occurred within a brief time interval. The circumstances vary from product to product, but are always based upon sensing of conditions caused by faults followed shortly by voltage losses. Sectionalizers are designed to coordinate with the operation of the circuit""s protective devices. Typical sectionalizers are devices such as the Cooper Power Systems Sectionalizer type GV or GW manufactured by Cooper Industries, Inc, or the EnergyLine Systems Model 2801-SC Switch Control manufactured by EnergyLine Systems. These are all well-known devices within the industry, and thus need not be described in detail herein.
Although utility acceptance of more sophisticated automation solutions to fault isolation and reconfiguration has been limited, many methods have been developed and marketed. The most primitive methods have typically involved placing control equipment and switchgear at strategic points in the power distribution grid and coordinating their operation entirely with the use of circuit parameters sensed and operated on locally and independently at each point. An example system of this type is the Kearney FILS system. More sophisticated methods have been developed for isolating/reconfiguring these circuits by communicating information sensed locally at the strategic points to a designated, higher level control entity(s). Example methods of this type are disclosed in U.S. Pat. Nos. 5,513,061 and 5,701,226 (Gelbein) and 5,341,268 (Ishiguro). Utilizing intelligent, distributed control methodologies, several methods have been developed to isolate/reconfigure distribution circuits without the need for the higher-level control entity(s). In systems implementing these methods, information is sensed and processed locally, acted on as much as possible locally, and then shared with other cooperating devices to either direct or enhance their ability to take action. Examples of these methods can be found in U.S. Pat. Nos. 3,970,898 (Baumann) and 5,784,237 (Velez), and in a prior version of EnergyLine Systems IntelliTEAM (Reg. T.M.) product and related U.S. patent application Ser. No. 08/978,966 (Nelson et al).
Most of these methods and systems contain significant restrictions on the power distribution equipment types and topologies supported. For example, Baumann, Velez, and Gelbein disclose methodologies tailored to non-fault break sectionalizing switches with breakers or reclosers only at the sources of supply. Thus, methodologies for integrating substation breakers, line reclosers, sectionalizers, and other equipment into generalized, automatic circuit reconfiguration systems have been limited. There are numerous reasons for this, related primarily to the nature of electric distribution systems:
1. Without communication equipment, it is difficult, if not impossible to coordinate the protective and fault isolation functions of more than two or three devices.
2. Communication equipment is costly or limited in capability, and the techniques for managing the information flow and sequence of events are primitive. This also adds labor to the setup and support of such systems.
3. The load density/diversity, differing wire sizes and intermixed construction techniques (overhead/underground) and inherently unpredictable loading patterns greatly complicate the automation of emergency switching decisions.
4. Generally, there is more than one alternate source of supply, but the source may have limited capacity to supply the feeder. This requires a more complex decision making process.
5. Even when there is only one alternate supply, and that supply is fortuitously placed at the end of the line, limitations on the current carrying capacity of the main feeder can limit the reconfiguration process.
6. The limited training and background of emergency crews requires that the equipment be easily operated in both automatic and manual operating modes.
7. The technology of protective relaying and reclosers has advanced to incorporate microprocessor-based technologies, and existing reconfiguration system solutions incorporating reclosers do not take advantage of the advanced capabilities of the microprocessor governed devices.
Examples of recent improvements in recloser technology include the Form 4c and Form 5c Recloser Controls manufactured by Cooper Industries, the SEL 351R Recloser Control manufactured by Schweitzer Engineering Laboratories, Inc. and the N, U, and W Series Recloser Controls manufactured by Nu-Lec Pt. Ltd. These products are capable of internally maintaining at least two separate sets of protective relay settings, selectable by the customer at the front panel or over communications. These sets of settings can be loosely referred-to as xe2x80x9cprofilesxe2x80x9d of protection characteristics, and may include a wide variety of selections including operating modes, protection features enabled, and level settings. In the case of the SEL 351R, the capability exists to modify profile settings based upon a procedural language and communication with external devices, although the specific methodology and details of doing this are left to the end user. A key attribute of these profiles is the amount of load and distance (or xe2x80x9creachxe2x80x9d) down the distribution line that can accommodated with reliable detection of the overcurrent fault.
A primary aspect of the present invention is to provide methodology and related system apparatus for using and coordinating the use of information conveyed over communications to dynamically modify the protection characteristics of distribution devices (including but not limited to substation breakers, reclosing substation breakers, and line reclosers). In this way, overall protection and reconfigurability of the distribution system or xe2x80x9cteamxe2x80x9d is greatly enhanced.
In another aspect of the invention, devices within the system in accordance with the present invention recognize the existence of cooperating devices outside of the team""s domain of direct control, managing information from these devices such that more intelligent local decision making and inter-team coordination can be performed. This information may include logical status indications, control requests, analog values or other data as will be presented below.
These and other purposes and advantages of the present invention will become more apparent to those skilled in the art from the following detailed description in conjunction with the appended drawings.